Process for producing aluminum surface coatings



United States Patent PROCESS FOR PRODUCING ALUMINUM SURFACE COATINGSGeorge H. Pimbley, Inglewood, Calif, assignor to Turco Products, Inc.,Los Angeles, Calif., a corporation of California j N0 Drawing.Application August 14, 1958- Serial No. 754,935 I 19 Claims. (Cl.148-627) This application is a continuation-in-part of my copendingapplications Serial Nos. 550,825, filed December 5, 1955, now Patent No.2,868,679; 647,933, filed March 25, 1957; and 710,402, filed 'January22, 1958.

This invention relates to the art of chemically coating aluminumsurfaces, and particularly to the production of substantially colorlessand corrosion resistant coatings upon such surfaces. Aluminum articlescontaining such coatings are particularly adapted for exterior usagesuch as, for example, aluminum house siding. The invention is especiallyconcerned with novel procedure for producing improved substantiallycolorless surface coatings on aluminum surfaces. The term aluminum asemployed herein is intended to denote pure aluminum, commercial gradealuminum containing small amounts of other ma-' terials, and aluminumalloys, i.e., those in which aluminum predominates.

In the prior art many methods are known of producing chemical films onaluminum surfaces, some of which have been commercially advantageous andhave been widely practiced for many years past. Among these are theformation of artificial oxide coatings by means of electric current inchemical baths, the immersion of .clean aluminum surfaces in alkalinebaths containing oxidizing com pounds whereby oxide type coatings areproduced, and immersion in acidic baths containing an acid which attacksaluminum, together with other components effective to form a fixedintegral film or coating upon the surface, the coating produced by thelatter procedure being termed a conversion coating. It is the latterclass of coatings to which the present invention particularly'relates. Abrief description of this category of chemical coatings on aluminum isgiven below.

When an object having a surface of aluminum ,is cleaned free of greaseand other surface soil, and is then immersed in or otherwise contactedwith a solution containing anions of an acid which'attacks aluminum,such as hydrofluoric acid, together with dichromate and hydrogen ions, achemical reaction occurs which results in a fixed, integral film uponthe aluminum surface. The precise nature of this reaction, and of theresulting film is not known. Probably some of thealuminum is dissolved,bringing about an increase of pH at the interface between the aluminumsurface and bath liquid. The conversion coating that results is probablya gel-like amorphous complex of unknown state of chemical compositionand physical structure. Generally, these coatings are thin, smooth,non-crystalline, relatively adherent and flexible, moderately lustrous,and are of yellow color, with more or less red, blue and greenishiridescence. The exact shade of color and degree of lustre differgreatly, according to which alloy of aluminum is being processed andaccording to the details of the processing procedure.

Chemical conversion coatings of the kind described, and as known in theprior art, have been found sufi"1- ciently advantageous to warrantincreasing use by airplane manufacturers and other industrialfabricators of aluminum, as well as for building purposes. The pro- 2cedures for application of such coatings have certain attractivefeatures, including simplicity, low processing temperatures, briefcontact time and feasibility of spray application.

In many instances it is desirable to produce a colorless or almostJcolorless conversion coating without impairing the other properties ofthe coating, for example, its corrosion resistance, smear-proofness anduniformity.

One object of the invention is to remove color from aluminum conversioncoatings.

Another object is to produce substantially colorless coatings or filmson aluminum surfaces.

Another object is the provision of aluminum conversion coatings ofreduced coloration and having good corrosion resistance.

Yet another object is to provide a process for removing practically allof the color from aluminum conversion coatings without detrimentaleffects on the corrosion resistance of the coating.

A still further object is the provision of procedure for simple, rapidand inexpensive treatment of aluminum for .production of aluminumconversion coatings which are remove substantially all yellow andrelated coloration from conversion coated aluminum surfaces, whetherfreshly formed or previously produced, within conveniently short timeperiods and at relatively low temperature's, without detriment to theappearance of the conver sion coating or to its corrosion resistance andpaint bonding qualities. By the term colorless or substantiallycolorless coatings, I mean to include coatings having a very pale orbarely perceptible coloration as well as coatings which appearcolorless, i.e., are transparent to show the color of the underlyingaluminum without any substantial added tinctorial effect.

The yellow conversion coating possesses its predominant yellow colordue, it is believed, to retention of a yellow chromophor in the form ofhexavalent chromium compound in the film following the conversioncoating operation in the presence of hexavalent chromium ion. I havefound that certain soluble agents have the ability to penetrate theconversion film and to react chemically with the hexavalentchromium'present to discharge the yellow chromophor and formsubstantially colorless products. I shall hereafter refer to theseagents as color dis-,

charge'agents. No yellow color or effect of yellow color in the treatingsolution containing the color discharge agent is noticeable orperceptible, indicating a true chemical reaction between mycolordischarge agent and the yellow hexavalent chromium compound in theconversion film. The color discharge agents employed in the inventionfor this purpose are those which on forma= tion of a 2% solution of theagent and immersion therein of a colored conversion coating at atreatment temperature'of Fffor a period not more than 15 minutes,produces a visible discoloration, or removal of color, from theconversion coating. The color discharge agents utilized in the inventionare also preferably free of any tendency to form insoluble coloredcompounds with hexavalent chromium. 7

I prefer to employ such color discharge agents ac cording to theinvention, which lend themselves readily to convenient processingmethods, and perform their" 3 function within a time period of about 2to about minutes at a temperature only moderately above roomtemperature, preferably between about room temperature and 120 F.

I have found that color discharge agents having the properties notedabove and suitable for producing substantially colorless aluminumconversion coatings according to the invention, include inorganic andorganic compounds. Preferably I utilize certain classes and types ofcoior discharge agents having the properties noted above; These includesoluble sulphur-containing compounds, soluble phosphorous-containingcompounds, soluble ferrocyanides, soluble divalent and trivalentchromiurn compounds, hydroquinone, quinhydrone, and hydrazine compounds.Illustrative specific examples of the groups of compounds notedimmediately above are ammoniu n and alkali metal thiosulphates andsulphites, ammonium and alkali metal phosphites andhypophosphites,phosphorous acid, ammonium and alkali metal ferrocyanides, chromousoxalate, chromic citrate, hydrazine hydrate, hydrazine sulfate and othersoluble hydrazine salts, and the like. may be useful for the process ofthis invention but which are less preferred includesoluble .ferrouscompounds such as ferrous sulphate, and ammonium and alkali metalhypochlorites.

Preferred specific color discharge agents suitable for use in theinvention are as follows:

TABLE 1 Potassium ferrocyanide Sodium hypophosphite Sodium thiosulphateHydroquinone Chromic nitrate Hydrazine sulphate Phosphorous acid testsfor timed periods up to 700 hours, and were found.

to possess excellent corrosion resistance by such tests, fully equal tothat of a yellow conversion-coated surface not treated in theabove-color discharge baths.

In similar tests made with hydrazine sulphate and phosphorous acid, itwas found that hydrazine sulphate produced a somewhat better dischargeof the yellow conversion coating than in the case of the first fivecompoundsof Table 1, with fairly good corrosion resistance obtained inthe resulting decolorized film, and that phosphorous acid as colordischarge agent produced a color discharge comparable to the first fivecompounds of Table 1, with fair corrosion resistance of thesubstantially colorless film.

The nature of the chemical reaction between my color discharge agents isnot presently understood. Whatever the reaction may be, such agentsappear to form reaction products with the hexavalent chromium, which aresubstantially colorless or have a coloration much less perceptible thanthe original yellow color. As aforementioned this is indicated by myobservation that the solution of my discharge agent following treatmentof conversion coatings therein, remains substantially free ofany yellowcoloration.

In contrast to the chemicalaction-ofmydecolorizing agents on thehexavalent chromium compounds of the aluminum conversion coating, whenwater alone is em- Rlvrs ta th urp se th te .sq simr t le s Other groupsof compounds which out the hexavalentchromium, as indicated by theyellow coloration of the solution following treatment therein of theconversion coating. 7

By employment of the invention process, the color can be discharged fromthicker conversion coatings and in less time than in the case of the useof water alone, and the resulting decolorized coating according to theinvention is substantially unimpaired as regards corrosion resistance,whereas use of water under these conditions results in impairment of theconversion coating. On freshly formed coatings, the results employing mycolor discharge agents are generally superior to the use of water asregards removal of color from the yellow conversion coating, even whenemploying lower temperatures and shorter periods of treatment than inthe case of water, and the corrosion resistance of the resultingsubstantially colorless film utilizing my colordischarge agents is"fully comparable to that obtained when employing water' under thesecircumstances. preferred-- to carry out treatment withthe colordischarge. agents of the invention on a freshly formed conversioncoating for bestresults, satisfactory results can also be achieved onold conversion coatings, that is, those formed substantially prior tothe time of treatment. with my color discharge agents. This is notpossible using water alone. in the latter case, an inordinately longperiod of time and high treatment temperature are required to remove anysubstantial amount of coloration, andduring such prolonged period ofrigorous treatment using water alone, the conversion coating isimpaired.

A comparison of the effect of my color discharge agents on yellowaluminum conversion coatings is given in Table 2 below. In this table,color discharge efiects are given in degrees. First degree indicates astrong remaining irridescent color tinge, i.e. tinges of red, green orblue; second degree indicates moderate remaining iridescent color tingesof the above type; third degree indicates only slight remainingiridescence of this nature; and fourth degree indicates no perceptibleremaining color or residual'tinge. The comparison was made as against acontrol of distilled water, which produced a first to second degreecolor discharge. Tests were made using 2% aqueous solutions of theagents noted in Table 2, except in the case of phosphorous acid andhydrazine hydrate, wherein concentrations of 0.5% and 0.2% wereemployed, respectively. In each case the time of treat ment with thesolution of color discharge agent was 15 minutes and temperature oftreatment of about F.

Time between the conversion coating bath for production.

of the yellow conversion coating and treatment in the respective bathsof my color discharge agents was 15 minutes in eachcase.

TABLE 2 Color Discharge Agent: I g 1. Potassium ferrocyanide 3rd degree.2. Sodium hypophosphite do. 3. Sodium thiosulphate do. 4. Hydroquinonedo. 5. Phosphorous acid do. 6. Chromic nitrate do. 7. Hydrazine sulphate4th degree. 8. Hydrazine hydrate 3rd degree.

It is noted from Table 2 that among those compounds While it is anddichromate ions. Such baths may also include other 7 ions, such-ashexavalent chromium ions. A typical prior art conversion. coating bathcan ,be, for example, an aqueous solution containing sodium fluoride,potassium dichromate and an acid suchas nitricacid. For best results, Iprefer to employ the after treatment process of the invention inconjunction with the conversion coating compositions and processesof myabove pending applications, Serial Nos. 550,825 and 710,402.

The-conversion coating baths disclosed in Serial No. 550,825 includeaqueous solutions containing cations of beryllium, magnesium, calcium,strontium or barium. The entire disclosure of my copending application,Serial No. 550,825, with respect to the composition of such baths isincorporated herein by reference.

The conversion coating baths disclosed inSerial No. 710,402 includeaqueous solutions containing one or more than one of the cations ofzinc, cadmium or mercury. The entire disclosure of mycopendingapplicationSerial No. 710,402 with respect to the composition of suchbaths is incorporated herein by reference.

In'preferred practice of the invention, the aluminum e surfaces of thepan preferably arefirst cleaned free of grease and soils by conventionalmethods, using cleaning agents known to be effective and safe onaluminum. Cleaning residues are removed by rinsing with water,

usually hot water.

.If the aluminum part has an obstructive oxide coatin the surfacecoating isnext deoxidized. This step may or may not be employed,depending upon the particularab loy and stock of aluminum beingprocessed- Certain alloys, such as 6l-S usually bear a surface-skin ofoxide, of such a nature that it isresistant to the formation of theconversion film. In such cases the oxide film must be removed, after thecleaning operation, and before contact withthe conversion coatingsolution or bath. This oxide film removal can be accomplished by varioussolutions usually. containing a large proportion of compound having theS radical, a moderate propertion of hexavalent chromium compound, and asmall proportion of fluorine compound. For aluminum alloys havingconsiderable proportions of silicon, it is beneficial to include nitricacid in the deoxidizing treatment, either as an ingredient of thedeoxidizing bath or of a separate bath employed in conjunction with theformer. A deoxidizing solution which may be used is as follows:

Grams. Sodium bisulphate 57.5 Ammonium bifiuoride 1.0

Potassium dichromate 16.5 Water to make one liter.

The surfaces of the aluminum-articles are immersed-in or otherwisecontacted with the above solution for'say five to eight minutes, attemperatures in the approximate range of 75-90 F. The surfaces arethenthoroughly rinsed with cold water.

The work surfaces are then immersed in or otherwise contacted with theconversion coating solution or bath, which can be made up, for example,according to my above applications Serial Nos. 550,825 or 710,402. ,Thetemperature of the bath may range from about 75m 110 F., the preferredtemperature range being about 80-90 F. for dipping, and about 90-100 F.-for spray application. The time period of contact may range, forexample, from 30 seconds to 10 minutes; the preferred ranges are 2 to 5minutes for dipping, and 1 to 2 minutes for spraying.

- The decolorizing baths of my invention for producing substantiallycolorless conversion coatings maycontain from about 0.1 to about 100grams per liter of solution, preferably about 1 to about 50 grams perliter, and most desirably from about 2 to about 20 grams per liter, ofthe above described color discharge agents. The aluminum 6 part on whichthe color conversion coating is formed can be immersed in a solution of.the aforementioned color discharge agents for about 2 to 30 minutes,usually about 15 minutes. Such solutionsmay be used cold or preferablywarm, for example, from about to deg. F.,

i the temperature preferably not exceeding deg: F.

The work is then water rinsed and dried. If desired, an

A piece of 3003 aluminum was dipped in a conversion coatingbathformulated as follows for a period of 25 minutes, about 5 times thenormal conversion treating time:

. Grams Calcium chromate 7.94 Sodium fiuoborate 2.52 Sodium molybdate.74 Nitric acid, 40 deg. B. 7.34

Water to make 1 liter.

A very deep golden colored film was obtained on the aluminum part;

The film was rinsed with water and then dipped in a solution at about120 F. of water containing 0.5% hydrazine sulphate by weight. Theconversion coating was rendered, substantially colorless in about 4minutes. When exposed to salt spray the film showed no appreciableattack after 500 hours exposure, thus exhibiting satisfactory corrosionresistance.

. I Example 2 e The procedure of Example 1 was repeated in thre separateinstances, but substituting in each instance for the aqueoushydrazinesulphate solution of Example 1, 2% solutions in water of each ofpotassium ferrocyanide, sodium hy ophosphite, sodium thiosulphate andphosphorus acid. The phosphorous acid acted nearly as rapidly as thehydrazine sulphate of Example 1, and the other materials noted aboveacted somewhat less rapidly. All of these materials resulted in aconversion coating having only slight iridescence and all of the testsshowed good corrosion resistance when subjected to 500 hours of saltspray exposure. The corrosion resistance of the coatings employing thesematerials, particularly the ,ferrocyanide, hypophosphite andthiosulphate compounds was somewhat superior to the corrosion resistanceof the decolorized coating of Example 1.

Examples 1 and 2 show that even where a deep coloration is present inthe conversion coating, the color discharge agents of the inventionfunction in a short treating time to substantially discharge the colorof the coating without adversely affecting corrosion resistance.

Example 3 A conversion coating formulation consisting of the ingredientslisted below was prepared.

Grams Beryllium hydroxide, Be(OH) 3.27 Chromium trioxide, CrO 7.60Nitric acid, 40 deg. B. 0.84 Sodium fiuoborate, NaBF 3.78 Sodiummolybdate, Na MoO 1.12

Water. tomake 1 liter. v

I .A 2024 aluminum alloy was immersed in this"s o'lutijon for about5"minutes, producing a yellow CQ'I lVeIS iOn' coa ing.

4 The resulting coating was rinsed and then immersedin a solution of 2%by weight of hydroquinone at a temperature'of about 120 deg. F. forabout 15 minutes.

The part was then rinsed and dried, resulting in a subresistance.

Example 4 Aprocedure similar to Example 3 can be carried out except thatin place of hydroquinone, the same amount of chromic nitrate may bealternatively employed to obtain results similar to those of Example 3.

Example 5 A formulatio'n consisting of the ingredients listed below wasprepared.

Percent by weight ZnSiF .6H O 48.2 CrQ 40.0 Boric acid (powder) 11.8

Total 100.0

The above mixture when kept in closed glasscontainers remains as a dry,yellow-tan colored, free-flowing powder, the chromium trioxide appearingas small pieces of dark red-brown colo'r.

A processing bath was prepared by adding the above formulation to waterin an amount of 1 /2 ounces per gallon of solution. The bath had a pH of1.6 without of my color discharge agents employed in the workingsolutions, and also the time of treatment of the aluminum article insuch baths can vary, depending, for example, upon the particular agentemployed, the operating temperature of the solution, the nature andthickness of the conversion coatings formed and the nature of thealuminum article being treated, and especially on'the degree ofcoloration imparted to the coating in the'co'nversion coatin bath.

If desired, other materials may be included in the treatment bath, inaddition to the color discharge agents described above. Thus, foreXample,I may incorporate Wetting agents, .such as an .alkylphenolethylene oxide condensate, or an alkyl .aryl ..sulphonate, e.g. an,alkyl benzene sulfonate wherein the alkyl group contains from about 12to about 18 carbon atoms, insuch baths-toincrease the effectivenessthereof, although these additives are not required.

While the above procedures and solutions containing the reducing agentsof the invention have been mostly described in connection with treatmentof the aluminum parts by dipping application in such solutions or baths,the aluminum parts may also be treated by spraying, brushing or swabbingapplications of such solutions.

From the foregoing, it is seen that I have developed a process forrendering aluminum conversion coatings substantially colorless, while atthe same time enhancingthesmear-proofness, uniformity, thickness,toughness and corrosion resistanceof the coating. The; protectivecoatings of this invention are usefulwhere it isdesired i to retain thebright, lustrous, colorless appearance of metallic aluminum. Aluminum..conversian coatings treated by this process may be painted, or may becoated with clear organic films, orgiven other supplementary fi i he It7 e c ly im nt. tha he n t surfaces have adequate corrosion resistance,andwlnere apaint is to beappliedto the surface of -the .coating, the.coated surface should .alsohave adequate adhesive properties to firmlybond .with paints androthensupplementa'ry. finishes. My process producescoatings possessing these qualities:

Whilelhave described/particular embodiments of my invention for thepurpose of illustration, it should be understood .th'at .variousmodifications and adaptations thereof'may be made ,within the spirit ofthe invention as set forth in the appended claims.

JIclaim:

1. A process for producing a substantially colorless chemicallybondedcoating on an aluminum'a'rticle, which comprises forming a coloredconversion coating on said article, said coating containingaiyellowhexaval'ent chromium material, and contacting said coloredconversion coating ,w'ith'an aqueous solution consisting essentially ofasoluble agent capable of discharging the yellow color of said hexavalentchromium material present in the conversionifilm', said color dischargeagent reacting chemically. .with'the hexaval'ent chromium to form asubstantially colorless reaction product, and substantiallydischargingthe color from said conversion coating, said color dischargeagent being one capable of producing a visible discoloration from acolored conversion coatingwhen immersed in 21.2% aqueous solution ofsaid agentat a treatmenttemperatureof 120 F. for a period not more than'15 minutes.

2. A process as defined inclaim l,"wherein said'agent is a member of"the group consisting ofpsoluble sulfurcontaining compounds solublephosphoruscontaining compounds, soluble ferrocyanides, soluble divalentand trivalent chromium compounds, hydroq'uinone, quinhyd'rone' andhydrazines.

3. A process as defined in claim 1, wherein said agent is a member .ofthe group consisting of'alkali metal and ammoniumferrocyanides,.hypophosphites, thiosulphates, phosphorous ac'id, chromicnitrate, hydrazine sulphate and hydroquinone.

4. A process as defined in claim '1, wherein said agent is potassiumferrocyanide.

' 5. A process as definel in claiml, wherein said agent is sodiumhypophosphite.

6. A process as defined in claim 1, wherein said agent is sodiumthiosulphate.

7. A process as defined in claim 1, wherein said agent is hydroquinone.a

8; A process as defined in claim 1, wherein the amount of said agentemployed is between about 0.1 and about grams per liter of. solution.

9'. 'A process as defined in 'claim 1, wherein the amount of saidagentemployed isbetween about 1 and about 50 grams. per liter of solution.

" 10. A process for producing a substantially colorless chemicallybonded coating on an aluminum article which comprisestreatingsaidarticlewith an aqueous acid aluminumconversion,coating'i'bath, said .bathComprising a halogen-containing anionand an anioncontaininghexavalent'cchromium, forming a colored conversion coating onsaid'article, said conversion coating containing yellow hexavalent,chromium material, and contacting said colored conversion coating, withan aqueous solution consisting essentially dffafsolubleagent'capable ofdischargingthe yellow color; of said hexavalent chromium materialpresent in -the.conversion film, said color discharge agent reactingchemically with the hexavalent chromium to formga 'substantiallycolorlessreaction product, and substantially discharging the color-,fromsaid conversion coating, the amount of said agent employed beingbetweenabout 0.1 and about 100 grams per liter of solu tion, said colordischarge agent being one capable of producing a visible. discolorationfrom a colored conversion coating-1 when immersed ina "2% aqueoussolutionlo'fzsaidagent. at a treatment temperature of F. for. a. periodnot more tha'n'lS minutes.

11.:A p'rocesslas defined in claim 10, wherein said agent is.an..alkali-meta1 ferrocya'nide.

12. A process as defined in claim 10, wherein said agent is an alkalimetal thiosulphate.

13. A process as defined in claim 10, wherein said agent is an alkalimetal hypophosphite.

14. A process as defined in claim 10, wherein the amount of said agentemployed is about 2 to about 20 grams per liter of solution.

15. A process as defined in claim 10, wherein said agent is a member ofthe group consisting of alkali metal and ammonium ferrocyanides,hypophosphites, and thiosulphates, phosphorous acid, chromic nitrate,hydrazine sulphate and hydroquinone, and the amount of said agentemployed is about 2 to about 20 grams per liter of solution. v

16. A process for producing a substantially colorless 17. A process asdefined in claim 16, wherein said agent is a member of the groupconsisting of alkali metal and ammonium ferrocyanides, hypophosphites,and thio sulphates, phosphorous acid, chromic nitrate, hydrazinesulphate and hydroquinone, and the amount of said agent employed isabout 0.1 to about 100 grams per liter of solution.

zine sulfate, and substantially discharging the color from chemicallybonded coating on an aluminum article, which comprises treating saidarticle with an aqueous acid aluminum conversion coating bath, said bathcomprising a fluorine-containing anion, an anion containing hexavalentchromium, and a member of the group consisting of beryllium, magnesium,calcium, strontium and barium,

forming a colored conversion coating on said article, said conversioncoating containing a yellow hexavalent chromium material, and contactingsaid colored conversion coating with an aqueous solution consistingessentially of a soluble agent capable of discharging the yel-v lowcolor of said hexavalent chromium material present in the conversionfilm, said color discharge agent reacting chemically with the hexavalentchromium to form a substantially colorless reaction product, andsubstantially discharging the color from said conversion coating, theamount of said agent employed being between about 0.1 and about 100grams per liter of solution, said color discharge agent being onecapable of producing a visible discoloration from a colored conversioncoating when immersed in a 2% aqueous solution of said agent at atreatment temperature of 120 F. for a period not more than 15 minutes.

said conversion coating. I

19. A process for producing a substantially colorless chemically bondedcoating on an aluminum article which comprises treating said articlewith an aqueous acid aluminum conversion coating bath, said bathcomprising a halogen-containing anionand an anion containing hexavalentchromium, forming a colored conversion coating on said article, saidconversion coating containing yellow hexavalent chromium material, andcontacting said colored conversion coating with an aqueous solution ofcolor from said conversion coating, the amount of said member employedbeing between about 0.1 and about i 100 grams per liter of solution.

References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR PRODUCING A SUBSTANTIALLY COLORLESS CHEMICALLY BONDEDCOATING ON AN ALUMINUM ARTICLE, WHICH COMPRISES FORMING A COLOREDCONVERSION COATING ON SAID ARTICLE, SAID COATING CONTAINING A YELLOWHEXAVALENT CHROMIUM MATERIAL, AND CONTACTING SAID COLORED CONVERSIONCOATING WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF A SOLUBLEAGENT CAPABLE OF DISCHARGING THE YELLOW COLOR OF SAID HEXAVALENTCHROMIUM MATERIAL PRESENT IN THE CONVERSION FILM, SAID COLOR DISCHARGEAGENT REACTING CHEMICALLY WITH THE HEXAVALENT CHROMIUM TO FORM ASUBSTANTIALLY COLORLESS REACTION PRODCT, AND SUBSTANTIALLY DISCHARGINGTHE COLOR FROM SAID CONVERSION COATING, SAID COLOR DISCHARGE AGENT BEINGONE CAPABLE OF PRODUCING A VISIBLE DISCOLORATION FROM A COLOREDCONVERSION COATING WHEN IMMERSED IN A 2% AQUEOUS SOLUTION OF SAID AGENTAT A TREATMENT TEMPERATURE OF 120*F. FOR A PERIOD NOT MORE THAN 15MINUTES.